This invention relates to a method of lowering the resistivity of an n-type metal oxide semiconductor or a conductive material which comprises as its essential component at least one n-type metal oxide semiconductor.
As is known, it is possible to lower the resistivity of an n-type metal oxide semiconductor useful as a conductor material in various electric and electronic devices either by a valence control technique, i.e. doping of the metal oxide with a metal element whose valence number is larger than that of the metal in the oxide by one, or by heat treatment in vacuum to increase oxygen defects of the metal oxide utilizing the n-type characteristic of the oxide. In either of these conventional methods there is the necessity for establishment of a high temperature atmosphere or a high temperature high vacuum condition by the use of a precisely controllable electric furnace or a vacuum furnace of a complicated construction. Also it is necessary to strictly control the amount of a dopant. Besides, lowering of the resistivity of a white or lightly colored metal oxide semiconductor by doping is almost inevitably accompanied by a change in color of the metal oxide from the inherent color to a different and deeper color, and such discoloration becomes more significant as the amount of the dopant is increased. This phenomenon is quite undesirable from an industrial viewpoint.